If you think America’s obesity epidemic will pass you by because you are young and fit now, think again. In the past 30 years, the portion of the population that is obese has risen from 15 percent to 33 percent. Now the Centers for Disease Control and Prevention predicts that more than 42 percent of people will be obese by 2030 – not just overweight, but clinically obese, with a body mass index of more than 30. (A healthy BMI for men is around 19 or 20.) And scientists can only speculate about the percentage of people who will be overweight.

This new statistic, while frightening, isn’t a surprise to obesity experts who have watched and forecasted America’s growing levels of fatness for decades. And now experts are scrambling to understand why. Most blame the epidemic on a variety of factors that can include everything from more chemicals in personal-care products, furniture, and food to the lack of sidewalks on city streets and a proliferation of elevators in buildings. Among many leading authorities, a contentious debate exists over whether it’s the calories or the carbs in food that cause people, including those who work out every day, to pack on the pounds. Here, we examine five different perspectives from some of the country’s leading obesity experts.